In recent years, influences on environment (environmental pollution) by nitrogen oxide (NOx), sulfur oxide (SOx) and particulate substances in exhaust, gas from diesel powered automobiles have become problems. In order to reduce nitrogen oxide and sulfur oxide in the exhaust gas, quality standard to lower the sulfur content in diesel gas oil to not more than 0.05% by weight (500 ppm) is laid down. Under such circumstances, production of very highly desulfurized gas oil having further decreased sulfur content is desired.
As shown in FIG. 3, a gas oil used for diesel oil or the like is usually produced by a process comprising preheating a gas oil fraction together with hydrogen in a furnace before desulfurization, then introducing the gas oil fraction into a hydrodesulfurization unit containing a catalyst and hydrodesulfurizing the gas oil fraction. In order to further reduce the sulfur concentration in this process, it is necessary to make the reaction conditions severe, for example, the reaction temperature of the desulfurization reaction is raised or the reaction time is prolonged. However, if high-degree desulfurization is carried out under such severe reaction conditions, there are problems that coloring of the resulting product gas oil takes place, the catalyst life is shortened, and the catalyst volume is increased.
As a means to solve the problems, the present applicant has found that a low-sulfur content desulfurized gas oil having a sulfur content of about 0.05% by weight can be efficiently produced by a process comprising fractionating a starting gas oil into plural fractions, subjecting each fraction to desulfurization and then mixing the fractions, and the process has already been proposed (see Japanese Patent Publication No. 49873/1994).
Under such circumstances, the present applicant has earnestly studied a process for economically producing a gas oil having a further decreased sulfur content, and as a result, he has found that a highly desulfurized gas oil can be produced economically and efficiently by fractionating a starting gas oil into a low-boiling fraction and a high-boiling fraction, then treating each of the fractions in a specific H2/Oil ratio and using an unreacted portion of hydrogen, which has been used in the hydrodesulfurization of the high-boiling fraction, for the hydrodesulfurization of the low-boiling fraction. Thus, the present invention has been accomplished.
It is an object of the invention to provide a process for desulfurizing a gas oil fraction, by which the amount of the catalyst used can be decreased to thereby produce a desulfurized gas oil economically and efficiently, a desulfurized gas oil and an apparatus for desulfurizing a gas oil fraction.